This invention relates to a method and apparatus for the heat treatment of industrial wastes wherein the wastes are subjected, with the addition of an oxygenated combustion-supporting gas supplied thereto, to pyrolysis and/or combustion of at least some of their constituents. More particularly, this invention is concerned with regulating the oxygenated gas in response to sensed predetermined characteristics produced by the heat treatment, whereby these characteristics are maintained within desired ranges.
Heat treatment is applicable to various types of wastes having different states at ordinary temperature, such as solid (wood, paper, rubber, metals, mineral matter, etc.), pasty (residual slimes) or liquid (residual liquors, cutting oils, diluted solvents, etc.) wastes. Heat treatment is useful in recovering, in the form of residue or incombustible ashes, substances from the waste materials which are relatively readily combustible, and also in recovering, in the form of calories, the thermal energy which such waste materials contain. Optionally, certain incombustible substances, or substances which are combustible only with great difficulty, such as metals, can be recovered by heat treatment of industrial wastes, which substances then can be recycled economically.
At present, the heat treatment of industrial wastes is carried out in installations consisting essentially of an incinerating furnace and a heat receiver, the latter receiving the flue gases from the furnace to recover the calories which such gases contain. The furnace generally comprises a pyrolysis chamber for the wastes and a combustion chamber for the flue gases. Following combustion, the flue gases enter the heat receiver wherein the thermal energy thereof is utilized. Typically, the furnace may be of the rotary type or of the stationary type having a grate. The receiver may be, for example, a steam-generating boiler supplying a turboalternator, a heat exchanger, or the like. The heat of the flue gases vaporizes or heats a mass of water stored in the boiler or a heat-transfer fluid circulating in the exchanger.
The energy output obtained from a combustible product will be greater if combustion is complete or nearly complete. Unfortunately, the conditions for nearly complete combustion, which result in a residue having zero heating value, are more difficult to define and to maintain when wastes are burned than when a conventional fluel is burned. In view of the extreme diversity of the composition of such wastes, and hence of their heating value, the criteria of combustion should be controlled as a function of the composition of an oxygenated combustion-supporting gas and the rate at which it is supplied to the furnace. Optionally, a makeup fuel may be used whenever the heating value of the wastes is too low. Moreover, the economic value of some of the constituents of these wastes, and hence the interest in recovering them, adds to the factors which must be considered for obtaining "good combustion", that is, combustion with maximum output but not destructive to certain recoverable constituents and not damaging to the furnace.
Processes heretofore known are carried out in treating installations wherein the oxygenated gas is atmospheric air blown into the chambers of the furnace by blowers. These processes do not readily allow the various criteria for good combustion to be satisfied, and result in considerable waste, such as residues of combustion which exhibit appreciable heating value due to underoxygenation or too low a feed rate of the oxygenated gas (air), or to an excessive feed rate of the air. Another disadvantage of such processes is their use of excessive makeup fuel, entailing excessive combustion temperatures that result in the destruction of certain useful constituents or in damage to the furnace walls, or the like.